Light-receiving device having light-trapping sheet

ABSTRACT

A light-receiving device of the present disclosure includes a light-trapping sheet, and a photoelectric conversion section optically coupled thereto. The light-trapping sheet includes: a light-transmitting sheet; and a plurality of light-coupling structures arranged in an inner portion of the light-transmitting sheet. The light-coupling structure includes first, second and third light-transmitting layers. A refractive index of the first and second light-transmitting layers is smaller than that of the light-transmitting sheet; and a refractive index of the third light-transmitting layer is larger than those of the first and second light-transmitting layers. The third light-transmitting layer has a diffraction grating parallel to the light-transmitting sheet. At least a part of the photoelectric conversion section is located along an outer edge of at least one of the surfaces of the light-transmitting sheet.

This is a continuation of International Application No. PCT/JP2012/007081, with an international filing date of Nov. 5, 2012, which claims priority of Japanese Patent Application No. 2011-244602, filed on Nov. 8, 2011, the contents of which are hereby incorporated by reference.

BACKGROUND

1. Technical Field

The present disclosure relates to a light-receiving device having a light-trapping sheet for allowing light-trapping utilizing diffraction.

2. Description of the Related Art

Where light is propagated between two light-propagating media of different refractive indices, since there is transmission and reflection of light at the interface, it is typically difficult to transfer, with a high efficiency, light from one light-propagating medium to the other light-propagating medium and maintain this state. A conventional grating coupling method shown in “Optical Integrated Circuits”, p 94, p 243, Hiroshi Nishihara, et al. Ohmsha Ltd., for example, can be mentioned as a technique for taking light into a transparent sheet from an environmental medium such as the air. FIGS. 23A and 23B are diagrams illustrating the principle of the grating coupling method, showing a cross-sectional view and a plan view of a light-transmitting layer 20 with a linear grating of a pitch Λ provided on a surface thereof. As shown in FIG. 23A, if light 23 a of a wavelength λ is allowed to enter the grating at a particular angle of incidence θ, it can be coupled to guided light 23B propagating inside the light-transmitting layer 20.

SUMMARY

However, according to the method disclosed in “Optical Integrated Circuits”, p 94, p 243, Hiroshi Nishihara, et al. Ohmsha Ltd., only light that satisfies predetermined conditions can be taken into the light-transmitting layer 20, and light that falls out of the conditions is not taken in.

An embodiment of the present disclosure provides a light-receiving device having a light-trapping sheet.

In one general aspect, a light-receiving device of the present disclosure includes a light-trapping sheet, and a photoelectric conversion section optically coupled to the light-trapping sheet, the light-trapping sheet including: a light-transmitting sheet having first and second principal surfaces; and a plurality of light-coupling structures arranged in an inner portion of the light-transmitting sheet at a first and second distance from the first and second principal surfaces, respectively. Each of the plurality of light-coupling structures includes a first light-transmitting layer, a second light-transmitting layer, and a third light-transmitting layer sandwiched therebetween; and a refractive index of the first and second light-transmitting layers is smaller than a refractive index of the light-transmitting sheet; a refractive index of the third light-transmitting layer is larger than the refractive index of the first and second light-transmitting layers; and the third light-transmitting layer has a diffraction grating parallel to the first and second principal surfaces of the light-transmitting sheet. At least a part of the photoelectric conversion section is located along an outer edge of at least one of the first and second principal surfaces of the light-transmitting sheet.

According to an embodiment of the present disclosure, it is possible to efficiently perform photoelectric conversion of light that has been taken in by utilizing total reflection of light.

Additional benefits and advantages of the disclosed embodiments will be apparent from the specification and Figures. The benefits and/or advantages may be individually provided by the various embodiments and features of the specification and drawings disclosure, and need not all be provided in order to obtain one or more of the same.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic cross-sectional view showing a first embodiment of a light-trapping sheet according to the present disclosure, and FIG. 1B is a plan view showing the position of a fourth area in the first embodiment.

FIGS. 2A and 2B are a schematic cross-sectional view and a plan view showing a light-coupling structure of the first embodiment, FIG. 2C is a cross-sectional view showing light being incident on an end face of the light-coupling structure, FIG. 2D is a cross-sectional view showing light being incident on the light-coupling structure with a light-transmitting layer 30 removed, and FIG. 2E is a cross-sectional view showing another configuration example of a light-coupling structure.

FIG. 3 is a cross-sectional view showing a structure used in analyzing the light-trapping sheet of the first embodiment.

FIGS. 4A to 4D show results of an analysis conducted using the structure shown in FIG. 3, wherein FIG. 4A to FIG. 4C each show the relationship between the angle of incidence of light and the transmittance thereof out of the sheet, and 4D shows the relationship between the groove depth of the diffraction grating and the light take-out efficiency out of the sheet.

FIG. 5A to 5E are diagrams showing light intensity distributions on the sheet cross section under conditions at positions indicated by arrows in FIGS. 4A to 4C.

FIGS. 6A to 6D show results of an analysis with the structure shown in FIG. 3 where the refractive index of a first light-transmitting layer 3 a and a second light-transmitting layer 3 b is made equal to the refractive index of the light-transmitting sheet, and the refractive index of the third light-transmitting layer 3 c is set to 2.0, wherein FIG. 6A to 6C show the relationship between the angle of incidence and the transmittance thereof out of the sheet, and D shows the relationship between the groove depth of the diffraction grating and the light take-out efficiency out of the sheet.

FIGS. 7A to 7E are schematic cross-sectional views showing a manufacturing procedure of the light-trapping sheet of the first embodiment.

FIGS. 8A and 8B are schematic plan views each showing a surface pattern of a mold used in manufacturing the light-trapping sheet of the first embodiment.

FIGS. 9A and 9B are a schematic cross-sectional view and a plan view showing a light-coupling structure used in a second embodiment of a light-trapping sheet according to the present disclosure.

FIG. 10 is a cross-sectional view showing a structure used in analyzing the light-trapping sheet of the second embodiment.

FIGS. 11A to 11D show results of an analysis conducted using the structure shown in FIG. 10, wherein FIGS. 11A to 11C show the relationship between the angle of incidence and the transmittance out of the sheet, and FIG. 11D shows the relationship between the groove depth of the diffraction grating and the light take-out efficiency out of the sheet.

FIGS. 12A to 12C show results of an analysis conducted using the structures shown in FIGS. 3 and 10 where the position of the light source is shifted by 5 μm in the x-axis negative direction, wherein FIGS. 12A to 12C each show the relationship between the angle of incidence of light on the end face of a single light-coupling structure and the transmittance thereof out of the sheet.

FIGS. 13A to 13E are schematic cross-sectional views showing a manufacturing procedure of the light-trapping sheet of the second embodiment.

FIGS. 14A and 14B are a schematic cross-sectional view and a plan view showing a light-coupling structure used in a third embodiment of a light-trapping sheet according to the present disclosure.

FIG. 15 is a cross-sectional view showing a structure used in analyzing the light-trapping sheet of the third embodiment.

FIGS. 16A to 16D show results of an analysis conducted using the structure shown in FIG. 15, wherein FIGS. 16A to 16C each show the relationship between the angle of incidence and the transmittance out of the sheet, and FIG. 16D shows the relationship between the groove depth of the diffraction grating and the light take-out efficiency out of the sheet.

FIGS. 17A to 17C show results of an analysis conducted using the structures shown in FIGS. 3 and 15 where the position of the light source is shifted by 5 μm in the x-axis negative direction, wherein FIGS. 17A to 17C show the relationship between the angle of incidence of light on the end face of a single light-coupling structure and the transmittance thereof out of the sheet.

FIGS. 18A to 18F are schematic cross-sectional views showing a manufacturing procedure of the light-trapping sheet of the third embodiment.

FIGS. 19A and 19B are schematic plan views each showing a surface pattern of a mold used in manufacturing the light-trapping sheet of the third embodiment.

FIG. 20A is a schematic cross-sectional view showing an embodiment of a light-receiving device according to the present disclosure, and FIGS. 20B and 20C are diagrams each illustrating an arrangement of a photoelectric conversion section.

FIG. 21 is a schematic cross-sectional view showing another embodiment of a light-receiving device according to the present disclosure.

FIG. 22 is a schematic cross-sectional view showing another embodiment of a light-receiving device according to the present disclosure.

FIGS. 23A and 23B are a cross-sectional view and a plan view of a linear grating for taking in light by a grating coupling method, and FIGS. 23C and 23D are diagrams showing the principle of the grating coupling method.

FIGS. 24A and 24B are schematic cross-sectional views showing still other embodiments of the light-trapping sheet according to the present disclosure.

FIG. 25A is a cross-sectional view showing a variation of the present embodiment, and FIGS. 25B and 25C are plan views showing variations.

DESCRIPTION OF EMBODIMENTS

First, thoughts by the present inventors on the problems with conventional techniques set forth above will be presented.

FIG. 23C shows a vector diagram of light incident on the grating provided on the light-transmitting layer 20. In FIG. 23C, circles 21 and 22 are centered about point O, wherein the radius of the circle 21 is equal to the refractive index n₀ of an environmental medium 1 surrounding the light-transmitting layer 20, and the radius of the circle is equal to the equivalent refractive index n_(eff) of the guided light 23B. The equivalent refractive index n_(eff) is dependent on the thickness of the light-transmitting layer 20, and takes a particular value, depending on the waveguide mode, between the refractive index n₀ of an environmental medium 1 and the refractive index n₁ of the light-transmitting layer 20. FIG. 23D shows a relationship between the effective thickness t_(eff) and the equivalent refractive index n_(eff) in a case where light propagates in the TE mode through the light-transmitting layer 20. The effective thickness is equal to the thickness of the light-transmitting layer 20 where there is no grating, and if there is a grating, it is the thickness of the light-transmitting layer 20 plus the average height of the grating. Induced guided light has modes such as zeroth, first, second, and so forth, which have different characteristic curves as shown in FIG. 23D. In FIG. 23C, point P is a point at which a line drawn from point O along the angle of incidence θ crosses the circle 21, point P′ is the foot of a perpendicular from point P to the x axis, and points Q and Q′ are points at which the circle 22 crosses the x axis. The condition for light coupling in the x-axis positive direction is represented by the length of P′Q being equal to an integral multiple of λ/Λ, and the condition for light coupling in the negative direction is represented by the length P′Q′ being equal to an integral multiple of λ/Λ. Note however that λ is the wavelength of light, and Λ is the pitch of the grating. That is, the condition for light coupling is represented by Expression 1.

$\begin{matrix} {\left\lbrack {{Exp}.\mspace{14mu} 1} \right\rbrack \mspace{661mu}} & \; \\ {{\sin \mspace{14mu} \theta} = {{\pm n_{eff}} + {q\frac{\lambda}{\Lambda}}}} & (1) \end{matrix}$

where q is the diffraction order represented by an integer. At an angle of incidence other than 0 defined by Expression 1, light is not coupled into the light-transmitting layer 20. Even with the same angle of incidence θ, light is not coupled for different wavelengths.

Note that as shown in FIG. 23B, for light 23 aa incident on the light-transmitting layer 20 at an azimuthal angle φ that is shifted by an angle φ from the direction of incidence of the light 23 a, the essential pitch of the grating of the light-transmitting layer 20 is Λ/cos φ. Therefore, for the light 23 a incident at a different azimuth, the condition for light coupling can be satisfied even with an angle of incidence θ and a wavelength that are different from those defined by Expression 1. That is, where changes in the azimuth of light incident on the light-transmitting layer 20 are tolerated, the condition for light coupling shown by Expression 1 is somewhat widened. However, incident light cannot be coupled to the guided light 23B over a wide wavelength range for every angle of incidence.

The guided light 23B, while propagating through the grating area, radiates light 23 b′ in the same direction as reflected light of the incident light 23 a. Therefore, even if light is incident at a position far away from an end portion 20 a of the grating and propagates through the light-transmitting layer 20 as the guided light 23B, it attenuates by the time it reaches the end portion 20 a of the grating. Therefore, only the light 23 a that is incident at a position close to the end portion 20 a of the grating can propagate through the light-transmitting layer 20 as the guided light 23B without being attenuated by the radiation. That is, even if the area of the grating is increased in order to couple a large amount of light, it is not possible to allow all the light incident on the grating to propagate as the guided light 23B.

With a light-receiving device according to an embodiment of the present disclosure, as opposed to the conventional technique described above, light incident on the light-transmitting sheet enters a light-coupling structure arranged in an inner portion thereof, and is converted by the diffraction grating of the third light-transmitting layer in the light-coupling structure to light that propagates in the direction along the third light-transmitting layer to be radiated from the end face of the light-coupling structure. Since the light-coupling structure is in such a positional relationship that it is parallel to the light-transmitting sheet surface, and light that is radiated from the light-coupling structure is repeatedly totally reflected between the surface of the light-transmitting sheet, and surfaces of other light-coupling structures, to be confined within the light-transmitting sheet. Since at least a part of a photoelectric conversion section is located along an outer edge of at least one of the first and second principal surfaces of the light-transmitting sheet, it is possible to efficiently perform photoelectric conversion of light that has been taken in.

A light-receiving device of the present disclosure includes a light-trapping sheet, and a photoelectric conversion section optically coupled to the light-trapping sheet. Now, before describing embodiments of the light-receiving device in detail, the light-trapping sheet will be first described in detail.

First Embodiment

A first embodiment of a light-trapping sheet according to the present disclosure will be described. FIG. 1A is a schematic cross-sectional view of a light-trapping sheet 51. The light-trapping sheet 51 includes a light-transmitting sheet 2 having a first principal surface 2 p and a second principal surface 2 q, and a plurality of light-coupling structure 3 provided in the light-transmitting sheet 2.

The light-transmitting sheet 2 is formed by a transparent material that transmits light of a desired wavelength or within a desired wavelength range determined according to the application. For example, it is formed by a material that transmits visible light (wavelength: 0.4 μm or more and 0.7 μm or less). The thickness of the light-transmitting sheet 2 is about 0.03 mm to 1 mm, for example. There is no particular limitation on the size of the first principal surface 2 p and the second principal surface 2 q, and they each have an area determined according to the application.

A cover sheet 2 e is bonded on the light-transmitting sheet 2 with a spacer 2 d sandwiched therebetween. Therefore, most of the first principal surface 2 p of the light-transmitting sheet 2 is in contact with a buffer layer 2 f. The spacer 2 d is formed by a material having a lower refractive index than the light-transmitting sheet, such as an aerogel. Note that the cover sheet 2 e may be formed on the second principal surface 2 q of the light-transmitting sheet 2 or on both surfaces. The thickness of the cover sheet 2 e is about 0.1 mm to 1.0 mm, for example.

As shown in FIG. 1A, the light-coupling structures 3 are arranged in an inner portion of the light-transmitting sheet 2 at a first distance d1 or more and a second distance d2 or more from the first principal surface 2 p and the second principal surface 2 q, respectively. Therefore, in the light-transmitting sheet 2, the light-coupling structure 3 is not provided in a first area 2 a that is in contact with the first principal surface 2 p and has a thickness of the first distance d1, and in a second area 2 b that is in contact with the second principal surface 2 q and has a thickness of the second distance d2, and the light-coupling structure 3 is provided in a third area 2 c sandwiched between the first area 2 a and the second area 2 b.

The light-coupling structures 3 are three-dimensionally arranged in the third area 2 c of the light-transmitting sheet 2. Preferably, the light-coupling structures 3 are two-dimensionally arranged on a surface parallel to the first principal surface 2 p and the second principal surface 2 q, and a plurality of sets of the two-dimensionally-arranged light-coupling structures 3 are layered together in the thickness direction of the light-transmitting sheet 2. Herein, “parallel” does not need to be mathematically strictly parallel. The term “parallel” as used in the present specification is meant to include cases where the direction is inclined by 10 degrees or less with respect to the strictly parallel direction.

The light-coupling structures 3 are arranged with a predetermined density in the x,y-axis direction (in-plane direction) and the z-axis direction (thickness direction). For example, the density is 10 to 10³ per 1 mm in the x-axis direction, 10 to 10³ per 1 mm in the y-axis direction, and about 10 to 10³ per 1 mm in the z-axis direction. In order to efficiently take in light illuminating the entirety of the first principal surface 2 p and the second principal surface 2 q of the light-transmitting sheet 2, the density with which the light-coupling structures 3 are arranged in the x-axis direction of the light-transmitting sheet 2, that in the y-axis direction and that in the z-axis direction may be independent of one another and uniform. Note however that depending on the application or the distribution of light illuminating the first principal surface 2 p and the second principal surface 2 q of the light-transmitting sheet 2, the arrangement of the light-coupling structures 3 in the light-transmitting sheet 2 may not be uniform and may have a predetermined distribution.

FIGS. 2A and 2B are a cross-sectional view along the thickness direction of the light-coupling structure 3, and a plan view orthogonal thereto. The light-coupling structure 3 includes the first light-transmitting layer 3 a, the second light-transmitting layer 3 b, and the third light-transmitting layer 3 c sandwiched therebetween. That is, the first light-transmitting layer 3 a, the second light-transmitting layer 3 b and the third light-transmitting layer 3 c sandwiched therebetween are arranged next to each other in a direction perpendicular to the first and second principal surfaces. Herein, “perpendicular” does not need to be mathematically strictly perpendicular. The term “perpendicular” as used in the present specification is meant to include cases where the direction is inclined by 10 degrees or less with respect to the strictly perpendicular direction. The third light-transmitting layer 3 c includes a diffraction grating 3 d having a linear grating of the pitch Λ provided on the reference plane. The linear grating of the diffraction grating 3 d may be formed by protrusions/depressions provided at the interface between the third light-transmitting layer 3 c and the first light-transmitting layer 3 a or the second light-transmitting layer 3 b, or may be provided inside the third light-transmitting layer 3 c as shown in FIG. 2E. It may be a grating based on refractive index differences, instead of a grating with protrusions/depressions. In the light-coupling structure 3, the diffraction grating 3 d of the third light-transmitting layer 3 c is arranged in the light-transmitting sheet 2 so as to be parallel to the first principal surface 2 p and the second principal surface 2 q of the light-trapping sheet 51. Herein, the diffraction grating being parallel to the first principal surface 2 p and the second principal surface 2 q means that the reference plane on which the grating is provided is parallel to the first principal surface 2 p and the second principal surface 2 q.

In one embodiment, where a plurality of light-coupling structures 3 are arranged on a surface parallel to the first principal surface 2 p and the second principal surface 2 q, they are arranged so that at least the first light-transmitting layer 3 a and the second light-transmitting layer 3 b are spaced apart from each other. That is, where a plurality of light-coupling structures 3 include a first light-coupling structure and a second light-coupling structure that are two-dimensionally arranged next to each other on a surface parallel to the first and second principal surfaces (2 p, 2 q), the first and/or second light-transmitting layer (3 a, 3 b) of the first light-coupling structure is/are spaced apart from the first and/or second light-transmitting layer (3 a, 3 b) of the second light-coupling structure. Herein, the first and/or second light-transmitting layer (3 a, 3 b) of first light-coupling structure being spaced apart from the first and/or second light-transmitting layer (3 a, 3 b) of the second light-coupling structure means to include any of the following cases. That is, a case where the first light-transmitting layer 3 a of the first light-coupling structure and the first light-transmitting layer 3 a of the second light-coupling structure are spaced apart from each other; a case where the second light-transmitting layer 3 b of the first light-coupling structure and the second light-transmitting layer 3 b of the second light-coupling structure are spaced apart from each other; and a case where the first and second light-transmitting layers (3 a, 3 b) of the first light-coupling structure and the first and second light-transmitting layers (3 a, 3 b) of the second light-coupling structure are spaced apart from each other. The third light-transmitting layers 3 c may be arranged to be spaced apart from each other, or may be arranged to be continuous with each other. In order to facilitate the manufacturing process, the third light-transmitting layers 3 c may be arranged to be continuous with each other. That is, the third light-transmitting layer of the first light-coupling structure and the third light-transmitting layer of the second light-coupling structure may be continuous with each other.

Where a plurality of light-coupling structures 3 are arranged in the thickness direction of the light-transmitting sheet 2, they are arranged to be spaced apart from each other. For example, where the second light-transmitting layer of the second light-coupling structure is present above the first light-transmitting layer of the first light-coupling structure, the first light-transmitting layer of the first light-coupling structure and the second light-transmitting layer of the second light-coupling structure are arranged to be spaced apart from each other.

The thicknesses of the first light-transmitting layer 3 a, the second light-transmitting layer 3 b and the third light-transmitting layer 3 c are a, b and t, respectively, and the step (depth) of the linear diffraction grating of the third light-transmitting layer 3 c is d. The surface of the third light-transmitting layer 3 c is parallel to the first principal surface 2 p and the second principal surface 2 q of the light-transmitting sheet 2, and surfaces 3 p and 3 q of the first light-transmitting layer 3 a and the second light-transmitting layer 3 b that are located on the opposite side from the third light-transmitting layer 3 c are also parallel to the first principal surface 2 p and the second principal surface 2 q of the light-transmitting sheet 2.

As will be described below, in order to be able to take in light of different wavelengths incident on the light-trapping sheet, the light-trapping sheet 51 may include a plurality of light-coupling structures 3, and at least two of the plurality of light-coupling structures may differ from each other in terms of the direction in which the diffraction grating 3 d extends. Alternatively, at least two of the plurality of light-coupling structures 3 may differ from each other in terms of the pitch Λ of the diffraction grating 3 d. Alternatively, a combination thereof may be used.

The refractive index of the first light-transmitting layer 3 a and the second light-transmitting layer 3 b is smaller than the refractive index of the light-transmitting sheet 2, and the refractive index of the third light-transmitting layer 3 c is larger than the refractive index of the first light-transmitting layer 3 a and the second light-transmitting layer 3 b. Hereinbelow, it is assumed that the first light-transmitting layer 3 a and the second light-transmitting layer 3 b are the air, and the refractive index thereof is 1. It is also assumed that the third light-transmitting layer 3 c is formed by the same medium as the light-transmitting sheet 2, and they have an equal refractive index.

The surfaces 3 p and 3 q of the first light-transmitting layer 3 a and the second light-transmitting layer 3 b of the light-coupling structure 3 are each a rectangular of which two sides are the lengths W and L, for example, and W and L are 3 μm or more and 100 μm or less. That is, the surfaces of the first light-transmitting layer 3 a and the second light-transmitting layer 3 b of the light-coupling structure 3 are each sized so as to circumscribe a circle having a diameter of 3 μm or more and 100 μm or less. The thickness (a+t+d+b) of the light-coupling structure 3 is 3 μm or less. While the surface (plane) of the light-coupling structure 3 has a rectangular shape as shown in FIG. 2B in the present embodiment, it may have a different shape, e.g., a polygonal shape, a circular shape, or an elliptical shape.

The light-trapping sheet 51 is used while being surrounded by an environmental medium. For example, the light-trapping sheet 51 is used in the air. In this case, the refractive index of the environmental medium is 1. Hereinbelow, the refractive index of the light-transmitting sheet 2 is assumed to be n_(s). Light 4 from the environmental medium passes through the cover sheet 2 e and the buffer layer 2 f, and enters the inside of the light-transmitting sheet 2 through the first principal surface 2 p and the second principal surface 2 q of the light-transmitting sheet 2. The buffer layer 2 f is formed by the same medium as the environmental medium, and the refractive index thereof is 1. The refractive index of the spacer 2 d is substantially equal to 1. An AR coat or anti-reflective nanostructures may be formed on the opposite surfaces of the cover sheet 2 e, the first principal surface 2 p and the second principal surface 2 q in order to increase the transmittance of the incident light 4. The anti-reflective nanostructures include minute protrusion/depression (or diffraction) structures, such as moth-eye structures, whose pitch and height are ⅓ or less the design wavelength. The design wavelength is the wavelength of light used when designing the various elements so that the light-trapping sheet 51 exhibits a predetermined function. Note that with anti-reflective nanostructures, Fresnel reflection is reduced but total reflection is present.

Hereinbelow, of the light present inside the light-transmitting sheet 2, light that satisfies sin θ<1/n_(s) will be referred to as the in-critical-angle light and light that satisfies sin θ≧1/n_(s) as the out-of-critical-angle light, regarding the angle θ (hereinafter referred to as the propagation angle) formed between the propagation azimuth thereof and the normal to the light-transmitting sheet 2 (a line perpendicular to the first principal surface 2 p and the second principal surface 2 q). In FIG. 1A, where in-critical-angle light 5 a is present inside the light-transmitting sheet 2, a portion thereof is converted by a light-coupling structure 3 to out-of-critical-angle light 5 b, and this light is totally reflected by the first principal surface 2 p to be out-of-critical-angle light 5 c that stays inside the sheet. A portion of the remaining in-critical-angle light 5 a′ of the in-critical-angle light 5 a is converted by another light-coupling structure 3 to out-of-critical-angle light 5 b′, and this light is reflected by the second principal surface 2 q to be out-of-critical-angle light 5 c′ that stays inside the sheet. Thus, all of the in-critical-angle light 5 a is converted to the out-of-critical-angle light 5 b or 5 b′ inside the third area 2 c where the light-coupling structures 3 are arranged.

On the other hand, where out-of-critical-angle light 6 a is present in the light-transmitting sheet 2, a portion thereof is totally reflected by the surface of a light-coupling structure 3 to be out-of-critical-angle light 6 b, and this light is totally reflected by the first principal surface 2 p to be out-of-critical-angle light 6 c that stays inside the sheet. A portion of the remaining light of the light 6 a becomes out-of-critical-angle light 6 b′ that passes through the third area 2 c where the light-coupling structures 3 are provided, and this light is totally reflected by the second principal surface 2 q to be out-of-critical-angle light 6 c′ that stays inside the light-transmitting sheet 2. Although not shown in the figure, there is also out-of-critical-angle light that stays inside the sheet while being totally reflected between different light-coupling structures 3 and between the first principal surface 2 p and the second principal surface 2 q, i.e., light that propagates through, while staying in, the first area 2 a, the second area 2 b or the third area 2 c. In this case, there may occur a deviation in the distribution of light propagating through the first area 2 a and the second area 2 b. Where the deviation in the distribution of light in the light-transmitting sheet 2 is problematic, one or more fourth area 2 h may be provided, in the third area 2 c in the light-transmitting sheet 2, where no light-coupling structure 3 is provided, as shown in FIG. 1A. That is, the light-coupling structures 3 are arranged only in the third area 2 c excluding the fourth area 2 h. In the light-transmitting sheet 2, the fourth area 2 h connects between the first area 2 a and the second area 2 b. The fourth area 2 h extends from the first area 2 a to the second area 2 b, or in the opposite direction, and the azimuth of an arbitrary straight line passing through the fourth area 2 h is along a larger angle than a critical angle that is defined by the refractive index of the light-transmitting sheet and the refractive index of the environmental medium around the light-transmitting sheet. That is, assuming that the refractive index of the environmental medium is 1 and the refractive index of the light-transmitting sheet 2 is n_(e), the angle θ′ of the direction 2 hx in which the arbitrary straight line passing through the fourth area 2 h extends with respect to the normal to the light-transmitting sheet 2 satisfies sin θ′≧1/n_(s). Herein, a straight line passing through the fourth area 2 h refers to the straight line penetrating the surface at which the fourth area 2 h is in contact with the first area 2 a and the surface at which the fourth area 2 h is in contact with the second area 2 b.

FIG. 1B is a plan view of the light-trapping sheet 51, showing the arrangement of the fourth areas 2 h. Preferably, a plurality of fourth areas 2 h are provided in the light-transmitting sheet 2 as shown in FIG. 1B. Since the fourth area 2 h extends from the first area 2 a to the second area 2 b, or in the opposite direction, at an angle larger than the critical angle, only out-of-critical-angle light, of the light propagating through the first area 2 a and the second area 2 b of the light-transmitting sheet 2, can pass from the first area 2 a to the second area 2 b, or in the opposite direction, passing through the fourth area 2 h. Therefore, it is possible to prevent the deviation of the light distribution in the light-trapping sheet 51.

As shown in FIG. 2A, the in-critical-angle light 5 a passes through the surface 3 q of the second light-transmitting layer 3 b, and a portion thereof is converted by the function of the diffraction grating 3 d to guided light 5B that propagates inside the third light-transmitting layer 3 c. The remainder primarily becomes the in-critical-angle light 5 a′ to pass through the light-coupling structure 3 as transmitted light or diffracted light, or becomes in-critical-angle light 5 r to pass through the light-coupling structure 3 as reflected light. Upon entering the second light-transmitting layer 3 b, there is also the out-of-critical-angle light 6 b which is reflected by the surface 3 q, but most of the light can be allowed to pass therethrough if anti-reflective nanostructures are formed on the surfaces 3 q and 3 p.

The coupling to the guided light 5B is the same as the principle of the conventional grating coupling method. Before the guided light 5B reaches an end face 3S of the third light-transmitting layer 3 c, a portion thereof is radiated in the same direction as the in-critical-angle light 5 r to be in-critical-angle light 5 r′, and the remainder is guided to be radiated from the end face 3S of the third light-transmitting layer 3 c to be the out-of-critical-angle light 5 c. On the other hand, the out-of-critical-angle light 6 a is totally reflected at the surface 3 q of the second light-transmitting layer 3 b, and it entirely becomes the out-of-critical-angle light 6 b. Thus, out-of-critical-angle light incident on the surface of the light-coupling structure (the surface 3 p of the first light-transmitting layer 3 a and the surface 3 q of the second light-transmitting layer 3 b) is reflected, as it is, as out-of-critical-angle light, while a portion of in-critical-angle light is converted to out-of-critical-angle light.

Note that if the length of the diffraction grating 3 d of the third light-transmitting layer 3 c is too long, the guided light 5B is entirely radiated before reaching the end face 3S. If it is too short, the efficiency of coupling to the guided light 5B is insufficient. How easily the guided light 5B is radiated is represented by the radiation loss coefficient α, and the intensity of the guided light 5B is multiplied by a factor of exp(−2αL) at a propagation distance of L. Assuming that the value of α is 10 (1/mm), the light intensity will be multiplied by a factor of 0.8 after propagation over 10 μm. The radiation loss coefficient α is related to the depth d of the diffraction grating 3 d, and it monotonously increases in the range of d≦d_(c) while being saturated in the range of d>d_(c). Where the wavelength of light is λ, the equivalent refractive index of the guided light 5B is n_(eff), the refractive index of the light-transmitting layer 3 c is n₁, and the duty of the diffraction grating 3 d (the ratio of the width of the protruding portion with respect to the pitch) is 0.5, d_(c) is give by Expression 2 below.

$\begin{matrix} {\left\lbrack {{Exp}.\mspace{14mu} 2} \right\rbrack \mspace{661mu}} & \; \\ {d_{c} \approx {\frac{\lambda}{2\pi}\sqrt{n_{eff}^{2} - \left( \frac{n_{1} - 1}{2} \right)^{2}}}} & (2) \end{matrix}$

For example, d_(c)=0.107 μm if λ=0.55 μm, n_(eff)=1.25, and n₁=1.5. In the monotonous increase region, the radiation loss coefficient α is in proportion to d squared. Therefore, the length of the diffraction grating 3 d, i.e., the length of the third light-transmitting layer 3 c (the dimensions W and L) is determined by the radiation loss coefficient α, and is dependent on the depth d of the diffraction grating 3 d. Assuming that by adjusting the depth d, the value of α is set in the range of 2 to 100 (1/mm) and the attenuation ratio to 0.5, W and L will be about 3 μm to 170 μm. Therefore, if W and L are 3 μm or more and 100 μm or less, as described above, it is possible to suppress the radiation loss to obtain a high coupling efficiency by adjusting the depth d.

Table 1 shows the visible light wavelength (λ=0.4 to 0.7 μm) of light that is coupled for the pitch Λ and the angle of incidence θ based on Expression 1, where the equivalent refractive index n_(eff) of the guided light 5B is set to 1.25. Each section of a dotted line is the range for coupling. For example, where the pitch is 0.4 μm, light having a wavelength of 0.4 μm is coupled at θ=−14° and light having a wavelength of 0.7 μm is coupled at θ=30°, thereby giving a visible light coupling range from θ=−14° to θ=30°.

TABLE 1 Angle of incidence θ (degrees) −90 −54 −33 −14 0 5 30 49 90 Pitch 0.18 0.4 Λ 0.20 0.4----0.5 (μm) 0.30 0.4----------------------0.7 0.40 0.4------------------0.7 0.56 0.4----------0.7 1.60 0.4----0.7 2.80 0.7

The polarity of the angle of incidence θ is relevant to the light coupling direction. Therefore, if one focuses only on the presence/absence of coupling while ignoring the light coupling direction, covering either the range of angles of incidence from 0 to 90° or from −90 to 0° means that coupling is achieved for every angle of incidence. Therefore, it can be seen from Table 1 that in order for light to be coupled for every visible light wavelength and for every angle of incidence, one may combine together light-coupling structures 3 including diffraction gratings 3 d having pitches Λ from 0.18 μm to 0.56 μm (from 0° to 90°), or from 0.30 μm to 2.80 μm (from −90° to 0°). Taking into consideration changes in the equivalent refractive index and manufacturing errors occurring when forming the waveguide layer and the diffraction grating, the pitch of the diffraction grating 3 d may be generally 0.1 μm or more and 3 μm or less.

For example, as shown in FIG. 28, the pitch of the diffraction grating 3 d is Λ for the in-critical-angle light 5 a that is incident in the direction perpendicular to the direction in which the diffraction grating 3 d extends, but the effective pitch of the diffraction grating 3 d for light 5 aa that is incident at an azimuthal angle of φ is Λ/cos φ. For example, where the azimuthal angle φ of incidence of the light 5 aa is 0 to 87°, the effective pitch is Λ to 19Λ. Therefore, where Λ=0.18 μm is set, it is possible to realize effective pitches Λ from 0.18 to 2.80 μm depending on the azimuth of incident light even with the same diffraction grating 3 d, and where Λ=0.30 μm is set, it is possible to realize pitches Λ from 0.30 to 2.80 μm. Therefore, it is possible to take in light for every visible light wavelength and for every angle of incidence also by placing light-coupling structures 3 of a single pitch in the light-transmitting sheet 2 while turning the light-coupling structures 3 so that the direction in which the diffraction grating extends (the azimuth of the diffraction grating) varies from 0° to 180°, other than by combining together light-coupling structures 3 including diffraction gratings 3 d having different pitches. Moreover, for a plurality of light-coupling structures 3, the pitch of the diffraction grating 3 d and the direction in which the diffraction grating 3 d extends may both be varied.

Next, light at end faces 3 r and 3 s perpendicular to the surfaces 3 p and 3 q of the light-coupling structure 3 (surfaces extending along the normal direction to the light-transmitting layer 3 b) will be discussed. As shown in FIG. 2C, possible courses of action for the light incident on the end face 3 r of the light-coupling structure 3 are: to be reflected by the end face 3 r; to be diffracted through the end face 3 r; to be refracted passing through the end face 3 r; and to be guided through the third light-transmitting layer 3 c passing through the end face 3 r. For example, the out-of-critical-angle light 6 a which is incident on, and passes through, the end faces of the first light-transmitting layer 3 a and the second light-transmitting layer 3 b is refracted to be in-critical-angle light 6 a′. A portion of light 6A which is incident on, and passes through, the end face of the third light-transmitting layer 3 c is converted to guided light 6B which propagates inside the third light-transmitting layer 3 c.

For reference, FIG. 2D shows the optical path obtained when the third light-transmitting layer 3 c is removed from the light-coupling structure 3 and the space left by the removal is filled with the same air as the first light-transmitting layer 3 a and the second light-transmitting layer 3 b. When the in-critical-angle light 5 a is incident on the surface 3 q of the light-coupling structure 3, if the position of incidence is close to the end face 3 s, it is output through the end face 3 s as the out-of-critical-angle light 5 a′ as a result of refraction. When the in-critical-angle light 5 a is incident on the end face 3 r of the light-coupling structure 3, it is totally reflected by the end face 3 r. When the out-of-critical-angle light 6 a is incident on the end face 3 r of the light-coupling structure 3, it is output from the surface 3 p as the in-critical-angle light 6 a′ as a result of refraction, irrespective of the position of incidence. When the out-of-critical-angle light 6 a is incident on the surface 3 q of the light-coupling structure 3, it is totally reflected by the surface 3 q.

Thus, for light that is incident on the end faces 3 r and 3 s of the light-coupling structure 3, the behavior is complicated, and even if out-of-critical-angle light is incident on the end face, it is not always output as out-of-critical-angle light. However, if the size of the surface (W, L) is set to be sufficiently (e.g., 4 times or more) larger than the size of the end face (a+t+d+b), the influence at the end face will be sufficiently small, and then the transmission or the reflection of light at the surfaces 3 p and 3 q can be seen as the transmission or reflection behavior of light for the entire light-coupling structure 3. Specifically, if the size of the surface 3 p of the first light-transmitting layer 3 a and the surface 3 q of the second light-transmitting layer 3 b is 4 times or more of the thickness of the light-coupling structure 3, it is possible to sufficiently ignore the influence of light at the end faces 3 r and 3 s of the light-coupling structure 3. Therefore, the light-coupling structures 3 exhibit a function of irreversibly converting in-critical-angle light to out-of-critical-angle light while maintaining out-of-critical-angle light as out-of-critical-angle light, and if the density of the light-coupling structures 3 is set to a sufficient density, it is possible to convert all the light incident on the light-trapping sheet 51 to out-of-critical-angle light (i.e., light confined within the sheet).

FIG. 3 shows a cross-sectional structure of a light-trapping sheet used in an analysis for confirming the light-confining effect of the light-trapping sheet 51. A light-trapping sheet including one light-coupling structure was used for the analysis. As shown in FIG. 3, a light source S (indicated by a broken line) having a width of 5 μm was set in parallel at a position of 1.7 μm from the second principal surface 2 q of the light-transmitting sheet 2, and the second light-transmitting layer 3 b having a width of 6 μm was arranged in parallel thereabove at a distance of 0.5 μm, with the third light-transmitting layer 3 c and the first light-transmitting layer 3 a of the same width being arranged thereabove. The first principal surface 2 p of the light-transmitting sheet 2 is located at a position of 2.5 μm from the surface of the first light-transmitting layer 3 a. The positions of the first light-transmitting layer 3 a, the second light-transmitting layer 3 b and the third light-transmitting layer 3 c are shifted side to side based on the angle θ so that a plane wave having a polarization at an angle of 45° with respect to the drawing sheet is output from the light source S at an azimuth forming the angle of θ with respect to the normal to the second principal surface 2 q, and the center of the incident light passes through the center of the surface of the second light-transmitting layer 3 b. The thickness a of the first light-transmitting layer 3 a was set to 0.3 μm, the thickness c of the second light-transmitting layer 3 b to 0.3 μm, the thickness t of the third light-transmitting layer 3 c to 0.4 μm, the depth d of the diffraction grating to 0.18 μm, and the pitch Λ of the diffraction grating to 0.36 μm. The refractive index of the light-transmitting sheet 2 and the third light-transmitting layer 3 c was assumed to be 1.5, and the refractive index of the environmental medium, the first light-transmitting layer 3 a and the second light-transmitting layer 3 b to be 1.0.

FIGS. 4A to 4C are results of an analysis using a light-trapping sheet having the structure shown in FIG. 3, each showing the relationship between the angle of incidence θ of light from the light source S incident on the light-coupling structure 3 and the transmittance of light that is output to the outside of the light-trapping sheet. The structure used in the analysis was as described above. A two-dimensional finite-difference time-domain method (FDTD) was used in the analysis. Therefore, the analysis results are those with a structure in which the cross section shown in FIG. 3 extends infinitely in the direction perpendicular to the drawings sheet. The transmittance was measured while it was stable, and was defined by the ratio of the integrated value of the Poynting vectors passing through the bottom surface (z=0 μm) and the top surface (z≈8 μm) of the analysis area with respect to the integrated value of the Poynting vectors passing through a closed curved surface surrounding the light source. While there are some calculation results exceeding 100%, it is because of slight errors in the measurement of the Poynting vectors of the light source. FIG. 4A shows the calculation results for a case where the wavelength λ of the light source is 0.45 μm, FIG. 4B for a case where the wavelength λ is 0.55 μm, and FIG. 4C for a case where the wavelength λ is 0.65 μm. Each figure uses the depth d of the diffraction grating as a parameter, and is also plotting the results obtained under a condition where there is no light-coupling structure 3 (a configuration only with the light-transmitting sheet 2 and the light source S).

A comparison between the results obtained in a case where the light-coupling structures 3 are present but the depth d of the diffraction grating is d=0 and the results (Nothing) obtained in a case where there is no light-coupling structure shows that the former has a lower transmittance than the latter in a range within the critical angle (41.8°), and they are both substantially zero for angles greater than or equal to that. The reason why the former has a lower transmittance within the critical angle is because light incident on the surface 3 q of the second light-transmitting layer 3 b is refracted and a portion thereof is output from the end face 3 s as out-of-critical-angle light, as described above with reference to FIG. 2D. Note however that for the former, out-of-critical-angle light entering through the end face 3 r of the light-coupling structure 3 is refracted through this surface, and is then refracted through the surface 3 p of the first light-transmitting layer 3 a to be in-critical-angle light inside the light-transmitting sheet 2, as described above again with reference to FIGS. 2C and 2D. Therefore, for a structure where d=0, there is conversion to out-of-critical-angle light while there is also conversion to in-critical-angle light, and it can be said that the light-confining effect as a whole is small.

On the other hand, a comparison between the results for a case where the depth of the grating is d=0.18 μm and the results for a case where d=0 shows that although the transmittance of the former is substantially close to that of the latter, the transmittance drops at positions indicated by arrows a, b, c, d and e. FIG. 4D shows the standard value (a value obtained by division by 90) of a value obtained by integrating each of the curves of FIGS. 4A, 4B and 4C for the angle of incidence θ, using the depth d of the diffraction grating as a parameter. Since the analysis model is two-dimensional, the integrated value is equal to the efficiency with which light in the light-confining sheet is taken out of the sheet. With any wavelength, the take-out efficiency decreases as d increases (at least for the comparison between d=0 and d=0.18). This represents the light-confining effect by a single light-coupling structure. This effect can be accumulated, and by increasing the number of light-coupling structures, it is possible to eventually confine all the light. Note that while this analysis is a two-dimensional model, there is always incident light that satisfies Expression 1, which is the coupling condition, for an arbitrary azimuthal angle φ shown in the plan view of FIG. 2A in an actual model (three-dimensional model), and therefore the transmittance curves shown in FIGS. 4A to 4D will drop for the entire range of the angle of incidence θ, rather than for the local range such as the arrows a, b, c, d and e, thus increasing the light-confining effect of the light-coupling structures.

FIGS. 5A to 5E show light intensity distribution diagrams in the light-trapping sheet under conditions indicated by arrows a, b, c, d and e of FIGS. 4A to 4D. Specifically, FIG. 5A shows the results where the wavelength is λ=0.45 μm and θ=5°, FIG. 5B shows the results where the wavelength is λ=0.55 μm and θ=0°, FIG. 5C shows the results where the wavelength is λ=0.55 μm and θ=10°, FIG. 5D shows the results where the wavelength is λ=0.65 μm and θ=10°, and FIG. 5E shows the results where the wavelength is λ=0.65 μm and θ=20°.

For the conditions and the angles of incidence shown in FIGS. 5A and 5B, since the refractive index of the third light-transmitting layer 3 c is higher than the refractive index of the first light-transmitting layer 3 a and the second light-transmitting layer 3 b surrounding the third light-transmitting layer 3 c, the third light-transmitting layer 3 c functions as a waveguide layer, and the incident light is coupled to the guided light propagating inside the third light-transmitting layer 3 c by the function of the diffraction grating, with the light being radiated into the light-transmitting sheet 2 from the end faces 3 r and 3 s of the third light-transmitting layer 3 c. The radiated light is out-of-critical-angle light, and is totally reflected by the first principal surface 2 p and the second principal surface 2 q of the light-transmitting sheet 2 to be confined within the light-transmitting sheet 2. Also for the conditions and the angles of incidence shown in FIGS. 5C, 5D and 5E, the incident light is coupled to the guided light propagating inside the third light-transmitting layer 3 c by the function of the diffraction grating, with the light being radiated into the sheet from the end face 3 r of the third light-transmitting layer 3 c. The radiated light is out-of-critical-angle light, and is totally reflected by the first principal surface 2 p and the second principal surface 2 q of the light-transmitting sheet 2 to be confined within the light-transmitting sheet 2. Note that in FIGS. 5A, 5C and 5E, the radiated light is divided into two, and the coupled light is guided light of the first-order mode whose phase is reversed above and below the cross section of the waveguide layer. On the other hand, in FIGS. 5B and 5D, the radiated light is in an undivided state, and the coupled light is guided light of the zeroth-order mode.

FIGS. 6A to 6D show results of an analysis using the structure shown in FIG. 3 where the refractive index of the first light-transmitting layer 3 a and the second light-transmitting layer 3 b is made to coincide with the refractive index of the light-transmitting sheet 2, and the refractive index of the third light-transmitting layer 3 c is changed to 2.0. The other conditions are the same as those when the analysis results shown in FIGS. 4A to 4D were obtained. FIG. 6A shows the results where the wavelength of the light source is λ=0.45 μm, FIG. 6B shows the results where the wavelength is λ=0.55 μm, and FIG. 6C shows the results where the wavelength is λ=0.65 μm. A comparison between the results where the depth of the grating is d=0.18 μm and the results where d=0 shows that the transmittance of the former drops at positions of arrows a, b, c, d, e and f, as compared with that of the latter. This is for the same reason as described above with reference to FIGS. 4A to 4D. However, in the region above the critical angle, the latter comes to the vicinity of zero whereas the former is substantially floating. This is because light of an angle of incidence above the critical angle diffracts through the diffraction grating of the light-coupling structure 3, and a portion thereof is converted to in-critical-angle light in the sheet. FIG. 6D shows the standard value (a value obtained by division by 90) of a value obtained by integrating each of the curves of FIGS. 6A, 6B and 6C for the angle of incidence θ, using the groove depth d as a parameter. For some conditions, an increase in d rather increased the take-out efficiency, thereby failing to obtain the light-confining effect. This indicates that the characteristics in the region above the critical angle cancel out the effects at the positions of the arrows a, b, c, d, e and f.

A comparison between analysis results of FIGS. 4 and 6 shows that the transmittance is successfully made zero above the critical angle in FIGS. 4A to 4D. A comparison between the results where the depth of the grating is d=0.18 μm and the results where d=0 shows there is no difference in the region above the critical angle, and they are both substantially zero. This is because the refractive index of the first light-transmitting layer 3 a and the second light-transmitting layer 3 b is set to be smaller than the refractive index of the light-transmitting sheet 2, resulting in total reflection at the surface 3 q which is the interface between the second light-transmitting layer 3 b and the light-transmitting sheet 2, whereby light of a large angle of incidence cannot enter the diffraction grating in the light-coupling structure 3, and there is no diffracted light caused by the diffraction grating. Thus, it can be seen that with the light-coupling structure 3, in order for the third light-transmitting layer 3 c to be a light guide layer, the refractive index thereof may be larger than the refractive index of the first light-transmitting layer 3 a and the second light-transmitting layer 3 b, and in order for out-of-critical-angle light not to enter the third light-transmitting layer 3 c, the refractive index of the first light-transmitting layer 3 a and the second light-transmitting layer 3 b may be smaller than the refractive index of the light-transmitting sheet 2. It can also be seen that in order to decrease the critical angle for the total reflection between the light-transmitting sheet 2 and the light-coupling structure, the difference between the refractive index of the first light-transmitting layer 3 a and the second light-transmitting layer 3 b and the refractive index of the light-transmitting sheet is preferably large, and the refractive index of the first light-transmitting layer 3 a and the second light-transmitting layer 3 b may be 1, for example.

Thus, with the light-trapping sheet of the present embodiment, light incident on the first principal surface and the second principal surface of the light-transmitting sheet at various angles becomes in-critical-angle light and enters a light-coupling structure arranged inside the light-transmitting sheet, and a portion thereof is converted by the diffraction grating in the light-coupling structure to guided light that propagates inside the third light-transmitting layer and is radiated from the end face of the light-coupling structure to be out-of-critical-angle light. Because the pitch of the diffraction grating varies and the azimuth of the diffraction grating varies from one light-coupling structure to another, this conversion is achieved for every azimuth over a wide wavelength range, e.g., over the entire visible light range. Since the length of the diffraction grating is short, it is possible to reduce the radiation loss of the guided light. Therefore, in-critical-angle light present inside the light-transmitting sheet is all converted to out-of-critical-angle light by a plurality of light-coupling structures. Since the refractive index of the first and second transmission layers of the light-coupling structure is smaller than the refractive index of the light-transmitting sheet, the out-of-critical-angle light is totally reflected by the surface of the light-coupling structure, and the light is repeatedly totally reflected between the surfaces of other light-coupling structures and the surface of the light-transmitting sheet, thus being confined within the light-transmitting sheet. Thus, the light-coupling structure irreversibly converts in-critical-angle light to out-of-critical-angle light, while maintaining out-of-critical-angle light in the out-of-critical-angle state. Therefore, if the density of the light-coupling structures is set to a sufficient density, it is possible to convert all the light incident on the light-trapping sheet to out-of-critical-angle light, i.e., light confined within the sheet.

Note that in FIG. 1A, the first principal surface 2 p of the light-transmitting sheet 2 is covered by the cover sheet 2 e via the buffer layer 2 f therebetween. Therefore, a foreign matter 2 g such as a drop of water remains on the surface of the cover sheet 2 e, and is prevented from coming into contact with the first principal surface 2 p. If the foreign matter 2 g comes into contact with the first principal surface 2 p, the total reflection relationship at the contact surface is lost, whereby the out-of-critical-angle light, which has been confined within the light-transmitting sheet 2, leaks to the outside via the foreign matter 2 g. Although the spacer 2 d is also in contact with the first principal surface 2 p, the refractive index thereof is substantially the same as the refractive index of the environmental medium, the total reflection relationship at the contact surface is maintained, and the out-of-critical-angle light will not leak to the outside via the spacer 2 d. If the surface area of the light-transmitting sheet is small, the buffer layer 2 f may be arranged between the cover sheet 2 e and the first principal surface 2 p, instead of providing the spacer 2 d sandwiched therebetween.

FIGS. 24A and 24B are cross-sectional views each showing an example arrangement of cover sheets 2 e. In the example of FIG. 24A, cover sheets 2 e are provided so as to oppose the first principal surface 2 p and the second principal surface 2 q of the light-transmitting sheet 2 both with a “gap” interposed therebetween. In this example, the first principal surface 2 p and the second principal surface are entirely covered by the cover sheets 2 e. In the example of FIG. 24B, a portion of the first principal surface 2 p of the light-transmitting sheet 2 is not opposing the cover sheet 2 e. Also in this example, a spacer 2 d is provided at a position other than the end portions of the second principal surface 2 q. Note that the “gap” described above may be filled with a fluid or a solid whose refractive index is sufficiently small.

The light-trapping sheet 51 can be manufactured by the following method, for example. FIGS. 7A to 7E are schematic cross-sectional configuration views showing a manufacturing procedure of the light-trapping sheet 51, and FIGS. 8A and 8B are schematic plan views each showing a pattern of a mold surface for producing the sheet.

In FIGS. 8A and 8B, rectangular minute structures 25A and 25B of the same size are two-dimensionally arranged, for example, on the surfaces of molds 25 a and 25 b. The arrangement of the minute structures 25A on the mold 25 a and the arrangement of the minute structures 25B on the mold 25 b are equal. In the present embodiment, the minute structures 25A and 25B are protrusions. The height of the minute structures 25A is the dimension b of FIG. 2A, and the height of the minute structures 25B is equivalent to the dimension a. While the surface of the minute structure 25B is a plane, a linear diffraction grating having a height of d and a pitch of Λ is formed on the surface of the minute structure 25A, and the azimuth of the diffraction grating (the direction in which the depressed portion or the protruding portion extends) varies from one minute structure 25A to another. While gratings of azimuths of 45° intervals, i.e., 0°, 45°, 90° and 135°, are arranged regularly in FIGS. 8A and 8B, gratings may be arranged in practice with an equal frequency at azimuths of smaller intervals, e.g., 30° or 15°.

As shown in FIG. 7A, with a thin layer of a spacer agent applied on the surface of the mold 25 b, a transparent resin sheet 24 is laid on the surface of the mold 25 b, and the mold 25 a is arranged on the sheet, pressing the resin sheet 24 sandwiched between the mold 25 b and the mold 25 b while the minute structures 25B and the minute structures 25A are aligned with each other.

As shown in FIG. 7B, the mold 25 a is lifted, thereby peeling the resin sheet 24 off the mold 25 b, and the resin sheet 24 is pressed against a resin sheet 24 a with a thin layer of an adhesive applied on the surface thereof as shown in FIG. 7C, thereby bonding together the resin sheet 24 and the resin sheet 24 a. As shown in FIG. 7D, an adhesive is applied in a thin layer on the bottom surface of the resin sheet 24 a, and it is pressed against similarly-formed resin sheets 24′ and 24′a while ignoring the alignment therebetween, thus bonding them together.

As shown in FIG. 7E, the mold 25 a is lifted while the resin sheet 24′a is secured, thereby peeling the resin sheets 24, 24 a, 24′ and 24′a as a whole off the mold 25 a.

Thereafter, the resin sheets 24, 24 a, 24′ and 24′a are replaced by the resin sheets 24′ and 24′a of FIG. 7D, and these steps are repeated, thereby producing the third area 2 c of the light-transmitting sheet 2 shown in FIG. 1A. Resin sheets to be the first area 2 a and the second area 2 b of the light-transmitting sheet 2 are bonded to the front surface and the reverse surface of the third area 2 c of the light-transmitting sheet 2, thereby completing the light-trapping sheet 51 shown in FIG. 1A. While an adhesive is used for the bonding between resin sheets in the present embodiment, the surfaces of the resin sheets may be heated so as to weld together the resin sheets, instead of using an adhesive. Anti-reflective nanostructures may be formed in advance on the surface of the resin sheet 24 a and the resin sheets to be the first area 2 a and the second area 2 b.

Second Embodiment

A second embodiment of a light-trapping sheet according to the present disclosure will be described. A light-trapping sheet 52 of the present embodiment is different from the light-coupling structure of the first embodiment in terms of the structure at the end face of the light-coupling structure. Therefore, the description hereinbelow will focus on the light-coupling structure of the present embodiment.

FIGS. 9A and 9B schematically show a cross-sectional structure and a planar structure of a light-coupling structure 3′ along the thickness direction of the light-trapping sheet 52. As shown in FIGS. 9A and 9B, a depressed portion 3 t having a depth of e is provided on the end faces 3 r and 3 s of the light-coupling structure 3′. The cross section of the depressed portion 3 t has a width that is tapered inwardly. Therefore, in the light-coupling structure 3′, the thickness of the first light-transmitting layer 3 a and that of the second light-transmitting layer 3 b decrease toward the outer edge side away from the center of the light-coupling structure 3′. The surfaces 3 p and 3 q are flat as they are in the first embodiment.

FIG. 10 shows a cross-sectional structure of a light-trapping sheet used in an analysis for confirming the light-confining effect of the light-trapping sheet 52 including the light-coupling structure 3′. The light-coupling structure and the light source are arranged at just the same positions as the corresponding elements in the structure used in the analysis in the first embodiment (FIG. 3).

FIGS. 11A to 11C show results of an analysis using a light-trapping sheet having the structure shown in FIG. 10, each showing the relationship between the angle of incidence θ of light from the light source S incident on the light-coupling structure 3′ and the transmittance of light that is output to the outside of the light-trapping sheet. The same method as that of the first embodiment was used for the analysis. FIG. 11A shows the results where the wavelength of the light source is λ=0.45 μm, FIG. 11B shows the results where the wavelength is λ=0.55 μm, and FIG. 11C shows the results where the wavelength is λ=0.65 μm. Each figure uses the depth d of the diffraction grating as a parameter, and is also plotting the results obtained under a condition where there is no light-coupling structure (a configuration only with the light-transmitting sheet 2 and the light source S).

A comparison between the results obtained in a case where the light-coupling structures 3′ are present but the depth of the diffraction grating is d=0 and the results (Nothing) obtained in a case where there is no light-coupling structure shows that the former is smaller than the latter in a range within the critical angle (41.8°), and they are both zero for angles greater than or equal to that. The reason why the former is smaller within the critical angle is because light incident on the surface 3 q of the second light-transmitting layer 3 b is refracted and a portion thereof is output from the right side face (the right side face of the third light-transmitting layer 3 c) as out-of-critical-angle light, as described above with reference to FIG. 2D.

On the other hand, a comparison between the results for a case where the depth of the grating is d=0.18 μm and the results for a case where d=0 shows that although the transmittance of the former is substantially close to that of the latter, the transmittance drops at positions indicated by arrows a, b, c, d and e. These positions correspond to conditions under which light is coupled to the guided light. FIG. 11D shows the standard value (a value obtained by division by 90) of a value obtained by integrating each of the curves of FIGS. 11A, 11B and 11C for the angle of incidence θ, using the groove depth d as a parameter. Since the analysis model is two-dimensional, the integrated value is equal to the efficiency with which light in the sheet is taken out of the sheet. With any wavelength, the take-out efficiency decreases as d increases (at least for the comparison between d=0 and d=0.18). This represents the light-confining effect by a single light-coupling structure, as with the analysis results in the first embodiment. This effect can be accumulated, and by increasing the number of light-coupling structures, it is possible to confine all the light. Note that while this analysis is a two-dimensional model, there is always incident light that satisfies Expression 1, which is the coupling condition, for an arbitrary azimuthal angle φ shown in the plan view of FIG. 2A in an actual three-dimensional model, and therefore the transmittance curves shown in FIGS. 11A to 11D will drop for the entire range of the angle of incidence 8, rather than for the local range such as the arrows a, b, c, d and e, thus increasing the light-confining effect of the light-coupling structures. The drops at positions of arrows b, c, d and e are smaller as compared with those of the analysis results of the first embodiment because the length of the grating (coupling length) is made smaller in the analysis model of this embodiment.

FIGS. 12A to 12C show results of an analysis of the second embodiment, each showing the relationship between the angle of incidence θ of light on the end face of a single light-coupling structure and the transmittance thereof out of the light-trapping sheet. In the analysis conditions used, only the position of the light source S is shifted by 5 μm in the x-axis negative direction from the conditions of FIG. 10 or FIG. 3. FIG. 12A shows a case where the wavelength of the light source is λ=0.45 μm, FIG. 12B a case where the wavelength is λ=0.55 μm, and FIG. 12C a case where the wavelength is λ=0.65 μm, wherein each figure shows a comparison between the model of this embodiment and the model of the first embodiment, and is also plotting the results obtained under a condition where there is no light-coupling structure (a configuration only with the light-transmitting sheet 2 and the light source S).

A comparison between the results for the model of the second embodiment and the results (Nothing) obtained in a case where there is no light-coupling structure shows that they substantially coincide with each other in both cases within the critical angle (41.8° or less), but the latter is substantially zero and the former substantially floats from zero outside the critical angle (41.8° or more). The former floats outside the critical angle because, as described above with reference to FIGS. 2C and 2D, light incident on the end face of the first light-transmitting layer 3 a and the second light-transmitting layer 3 b of the light-coupling structure refracts, and then becomes in-critical-angle light and is output from the first principal surface 2 p. In contrast, in the analysis results for the model of the second embodiment, the floating outside the critical angle is partially suppressed. This is because the first light-transmitting layer 3 a and the second light-transmitting layer 3 b account for no area on the end face of the second embodiment, and the refraction at the end face is somewhat suppressed. Therefore, the second embodiment is a configuration such that the influence at the end face (the phenomenon that out-of-critical-angle light is converted to in-critical-angle light) can be ignored more than in the first embodiment, and can be said to be a configuration having a greater light-confining effect. Note that in FIGS. 12A to 12C, the length of the light source is set to 5 μm. Increasing this length will increase the proportion of a component that that deviates from the end face of the light-coupling structure and is incident directly on the first principal surface 2 p to be totally reflected or is totally reflected at the surface 3 q of the light-coupling structure, thus reducing the floating outside the critical angle. If the length of the light source is set to 20 μm, which is 4 times more, while the light-coupling structure is set to be about 21 μm, only the floating outside the critical angle, of the end face incidence characteristics, is reduced to about ¼.

FIGS. 13A to 13E are schematic cross-sectional views showing an example of a production procedure for the light-trapping sheet 52 of the present embodiment. The light-trapping sheet 52 can be manufactured by using a similar procedure to that of the first embodiment, while providing slopes 25A′ and 25B′ at the outer edge portions of the minute structures 25A and 25B of the molds 25 a and 25 b. Except for the shapes of the molds 25 a and 25 b being different, the light-trapping sheet 52 of the present embodiment can be manufactured in a similar manner to the light-trapping sheet 51 of the first embodiment, and therefore the manufacturing procedure will not be described in detail.

Third Embodiment

A third embodiment of a light-trapping sheet according to the present disclosure will be described. A light-trapping sheet 53 of the present embodiment is different from the light-coupling structure of the second embodiment in terms of the structure at the end face of the light-coupling structure. Therefore, the description hereinbelow will focus on the light-coupling structure of the present embodiment.

FIGS. 14A and 14B schematically show a cross-sectional structure and a planar structure of a light-coupling structure 3″ along the thickness direction of the light-trapping sheet 53. As shown in FIGS. 14A and 14B, on the surfaces 3 p and 3 q of the light-coupling structure 3″, tapered portions 3 u and 3 v are provided across areas having the width e adjacent to the end faces 3 r and 3 s. Therefore, the thicknesses of the first light-transmitting layer 3 a and the second light-transmitting layer 3 b are decreased toward the outer edge side away from the center of the light-coupling structure 3″ while maintaining the flatness of the interface between the first light-transmitting layer 3 a and the second light-transmitting layer 3 b and the third light-transmitting layer 3 c.

FIG. 15 shows a cross-sectional structure of a light-trapping sheet used in the analysis for confirming the light-confining effect of the light-trapping sheet 53 including the light-coupling structure 3″. The light-coupling structure and the light source are provided at just the same positions as those in the structure used in the analysis in the first embodiment (FIG. 3).

FIGS. 16A to 16C show results of an analysis using a light-trapping sheet having the structure shown in FIG. 15, each showing the relationship between the angle of incidence θ of light from the light source S incident on the side of the light-coupling structure 3′ and the transmittance of light that is output to the outside of the light-trapping sheet. The same method as that of the first embodiment was used for the analysis. FIG. 16A is for a case where the wavelength of the light source is λ=0.45 μm, FIG. 16B for a case where the wavelength is λ=0.55 μm, and FIG. 16C for a case where the wavelength is λ=0.65 μm, wherein each figure uses the depth d of the diffraction grating as a parameter, and is also plotting the results obtained under a condition where there is no light-coupling structure (a configuration only with the light-transmitting sheet 2 and the light source S).

A comparison between the results obtained in a case where the light-coupling structures are present but the depth of the grating is d=0 and the results (Nothing) obtained in a case where there is no light-coupling structure shows that the former is smaller than the latter in a range within the critical angle (41.8°), and the latter is zero for angles greater than or equal to the critical angle, whereas floating remains for the former in the range up to 55°. The reason why the former is smaller within the critical angle is because light incident on the surface 3 q of the second light-transmitting layer 3 b is refracted and a portion thereof is output from the right side face (the right side face of the third light-transmitting layer 3 c) as out-of-critical-angle light, as described above with reference to FIG. 2D. There are two possible reasons for the former to float for angles greater than or equal to the critical angle. First, the surface 3 q of the second light-transmitting layer 3 b is sloped toward the outer edge portion, whereby a portion of light exceeding the critical angle can be incident on the surface 3 q of the second light-transmitting layer 3 b within the critical angle, and this light diffracts through the grating inside the light-coupling structure to be in-critical-angle light. Second, the thickness of the second light-transmitting layer 3 b is too small in the outer edge portion, and a portion of light exceeding the critical angle passes into the inside of the light-coupling structure in the form of evanescent light, and this light diffracts through the grating to be in-critical-angle light.

On the other hand, a comparison between the results for a case where the depth of the diffraction grating is d=0.18 μm and the results for a case where d=0 shows that although the transmittance of the former is substantially close to that of the latter, the transmittance drops at positions of arrows a, b, c, d and e. These positions correspond to conditions under which light is coupled to the guided light, and the light is guided, after which it is radiated from the end face of the third light-transmitting layer 3 c to be out-of-critical-angle light. This radiated light falls within the range of about ±35° about a propagation angle of 90° (x-axis direction) (see FIG. 5).

In FIGS. 16A to 16D, the floating of transmitted light is suppressed at the angle of incidence of 55° or more, and it becomes substantially zero, indicating that light to be guided light and radiated becomes out-of-critical-angle light (light whose propagation angle is 55° or more) that is repeatedly totally reflected and stays inside the sheet. Note that as the surface 3 p of the first light-transmitting layer 3 a and the surface 3 q of the second light-transmitting layer 3 b are sloped toward the outer edge portion, the propagation angle of light that is totally reflected at these surfaces increases and decreases depending on the slope direction, but since they occur with the same probability, it is possible to maintain substantially the same propagation angle as a whole.

FIG. 16D shows the standard value (a value obtained by division by 90) of a value obtained by integrating each of the curves of FIGS. 16A, 16B and 16C for the angle of incidence θ, using the groove depth d as a parameter. Since the analysis model is two-dimensional, the integrated value is equal to the efficiency with which light in the sheet is taken out the sheet. With any wavelength, the take-out efficiency decreases as d increases (at least for the comparison between d=0 and d=0.18). This represents the light-confining effect by a single light-coupling structure, as with the analysis results of the first embodiment. This effect can be accumulated, and by increasing the number of light-coupling structures, it is possible to confine all the light. Note that while this analysis is a two-dimensional model, there is always incident light that satisfies Expression 1, which is the coupling condition, for an arbitrary azimuthal angle φ shown in the plan view of FIG. 2A in an actual three-dimensional model, and therefore the transmittance curves shown in FIGS. 16A to 16D will drop for the entire range of the angle of incidence θ, rather than for the local range such as the arrows a, b, c, d and e, thus increasing the light-confining effect of the light-coupling structures.

FIGS. 17A to 17C show results of an analysis using the sheet of the third embodiment, each showing the relationship between the angle of incidence e of light on the end face of a single light-coupling structure and the transmittance thereof out of the light-trapping sheet. In the analysis conditions used, only the position of the light source S is shifted by 5 μm in the x-axis negative direction from the conditions of FIG. 15 or FIG. 3. FIG. 17A shows a case where the wavelength of the light source is λ=0.45 μm, FIG. 178 a case where the wavelength is λ=0.55 μm, and FIG. 17C a case where the wavelength is λ=0.65 μm, wherein each figure shows a comparison between the model of this embodiment and the model of Embodiment 1, and is also plotting the results obtained under a condition where there is no light-coupling structure (a configuration only with the light-transmitting sheet 2 and the light source S). A comparison between the results for the model of Embodiment 1 and the results (Nothing) obtained in a case where there is no light-coupling structure shows that they substantially coincide with each other in both cases within the critical angle (41.8° or less), but the latter is substantially zero and the former substantially floats outside the critical angle (41.8° or more). The former floats outside the critical angle because, as described above with reference to FIGS. 2C and 2D, light incident on the end face of the first light-transmitting layer 3 a and the second light-transmitting layer 3 b of the light-coupling structure refracts, and then becomes in-critical-angle light and is output from the upper surface. In contrast, with the results for the model of the third embodiment, the floating is significantly suppressed to substantially zero in the range where the angle of incidence is 55° or more. This is because the first light-transmitting layer 3 a and the second light-transmitting layer 3 b account for no area on the end face of the third embodiment, and a component that is supposed to refract through the end face is totally reflected at the sloped surface 3 q of the second light-transmitting layer 3 b. Therefore, the third embodiment is a configuration such that the influence at the end face (the phenomenon that out-of-critical-angle light is converted to in-critical-angle light) can be ignored more than in the first embodiment or the second embodiment, and can be said to be a configuration having a greater light-confining effect.

The light-trapping sheet 53 can be manufactured by the following method, for example. FIGS. 18A to 18F are schematic cross-sectional configuration views showing a manufacturing procedure of the light-trapping sheet 53, and FIGS. 8A and 8B are schematic plan views each showing a pattern of a mold surface for producing the sheet. In FIG. 19A, the surface of the mold 25 a is a plane, and rectangular minute structures 25A of the same size are two-dimensionally arranged, for example, on the surface of the mold 25 a. The rectangular minute structure 25A is a diffraction grating having a height of d and a pitch of Λ. The azimuth of the diffraction grating varies from one minute structure 25A to another. While diffraction gratings of 45°-interval azimuths, i.e., 0°, 45°, 90° and 135°, are arranged regularly in FIG. 19A, gratings may be arranged in practice with an equal frequency at smaller azimuths intervals, e.g., 30° or 15°. The rectangular minute structures 25B and 25B′ are two-dimensionally arranged also on the surfaces of the molds 25 b and 25 b′ of FIG. 19B. The pitch of the arrangement of the minute structures 25B and 25B′ is equal to the pitch of the arrangement of the minute structures 25A. The minute structures 25B and 25B′ are depressed portions with planar bottoms. The depth of the depressed portion is equivalent to the dimension a or b of FIGS. 14A and 14B. The minute structures 25A of the mold 25 a are so large that their square shapes are almost in contact with one another (they may be in contact with one another), the minute structures 25B and 25B′ of the molds 25 b and 25 b′ are smaller.

As shown in FIG. 18A, the transparent resin sheet 24 is laid on a mold 25 c having a flat surface and, with a thin layer of a spacer agent applied thereon, is pressed by the mold 25 a. As shown in FIG. 18B, the mold 25 a is lifted to peel the mold 25 a off the resin sheet, and the flat resin sheet 24 a is laid on the resin sheet 24, onto which a diffraction grating has been transferred.

As shown in FIG. 18C, the resin sheet 24 and the resin sheet 24 a are pressed by the mold 25 b while being heated, and the resin sheet 24 a is raised in the area of a depression 25B of the mold 25 b while attaching the resin sheet 24 and the resin sheet 24 a together in the other area. In this process, the diffraction grating is all buried to disappear in the attached portion, and remains only in the area where the resin sheet 24 a is raised. Raising the resin sheet 24 a forms an air layer (or a vacuum layer) between the resin sheet 24 a and the resin sheet 24. As shown in FIG. 18D, the mold 25 c is lifted to peel the mold 25 c off the resin sheet 24, and a resin sheet 24 a′ is laid under the resin sheet 24. As shown in FIG. 18E, the resin sheet 24 and the resin sheet 24 a′ are pressed by a mold 25 b′ while being heated, and the resin sheet 24 a′ is raised in the area of a depression 25B′ of the mold 25 b′ while attaching the resin sheet 24 and the resin sheet 24 a′ together in the other area. The rise of the resin sheet 24 a′ forms an air layer (or a vacuum layer) between the resin sheet 24 a′ and the resin sheet 24. As shown in FIG. 18F, the molds 25 b and 25 b′ are peeled off, completing an attached sheet of the resin sheet 24 a, the resin sheet 24 and the resin sheet 24 a′. Thereafter, these attached sheets are bonded together via an adhesive layer therebetween, and the process is repeated, thereby producing the third area 2 c of the light-transmitting sheet 2 shown in FIG. 1A. A resin sheet to be the first area 2 a and the second area 2 b of the light-transmitting sheet 2 is bonded to the front surface and the reverse surface of the third area 2 c of the light-transmitting sheet 2, thereby completing the light-trapping sheet 53. Note that anti-reflective nanostructures may be formed in advance on the surface of the resin sheet to be the resin sheets 24 a and 24 a′, the first area 2 a and the second area 2 b.

In embodiments hereinbelow, descriptions with respect to the cover sheets 2 e will, be omitted because they are the same as, and redundant with, those given in the first embodiment.

Fourth Embodiment

An embodiment of a light-receiving device according to the present disclosure will be described. FIG. 20A schematically shows a cross-sectional structure of a light-receiving device 54 of the present embodiment. The light-receiving device 54 includes the light-trapping sheet 51 of the first embodiment and a photoelectric conversion section 7. The light-trapping sheet 52 of the second embodiment or the light-trapping sheet 53 of the third embodiment may be used instead of the light-trapping sheet 51.

A reflective film 11 is preferably provided on end faces 2 s and 2 r of the light-trapping sheet 51. The photoelectric conversion section 7 is provided adjacent to the second principal surface 2 q of the light-trapping sheet 51. If the light-transmitting sheet 2 has a plurality of end faces, the reflective film 11 may be provided on all of the end faces. In the present embodiment, a portion of the second principal surface 2 q and a light-receiving portion of the photoelectric conversion section 7 are in contact with each other. The photoelectric conversion section 7 may be provided in a portion of the first principal surface 2 p of the light-trapping sheet 51.

By covering the end faces 2 r and 2 s of the light-trapping sheet 51 with the reflective film 11, light that has been taken and enclosed in the light-trapping sheet 51 will circulate in the light-trapping sheet 51.

The photoelectric conversion section 7 is a solar cell formed by a silicon. A plurality of photoelectric conversion sections 7 may be attached to one sheet of light-trapping sheet 51. Since the refractive index of silicon is about 5, even if light is made incident perpendicularly on the light-receiving surface of a solar cell, around 40% of the incident light is normally lost through reflection without being taken in the photoelectric conversion section 7. The reflection loss further increases when the light is incident diagonally. Although an AR coat or anti-reflective nanostructures are formed on the surface of a commercially-available solar cell in order to reduce the amount reflection, a sufficient level of performance has not been achieved. Moreover, a metal layer is present inside the solar cell, and a large portion of light that is reflected by the metal layer is radiated to the outside. With an AR coat or anti-reflective nanostructures, the reflected light is radiated to the outside with a high efficiency.

In contrast, the light-trapping sheet of the present disclosure takes in and encloses light for every visible light wavelength and for every angle of incidence in the light-trapping sheet. Therefore, with the light-receiving device 54, light entering through the first principal surface 2 p of the light-trapping sheet 51 is taken into the light-trapping sheet 51 and circulates in the light-trapping sheet 51. Since the refractive index of silicon is larger than the refractive index of the light-transmitting sheet 2, the out-of-critical-angle light 5 b′ and 6 b′ incident on the second principal surface 2 q are not totally reflected but portions thereof are transmitted into the photoelectric conversion section 7 as refracted light 5 d′ and 6 d′ and are converted to electric current in the photoelectric conversion section. After the reflected out-of-critical-angle light 5 c′ and 6 c′ propagate inside the photoelectric conversion section 7, they enter again and are used in photoelectric conversion until all the enclosed light is gone. Assuming that the refractive index of the transmissive sheet 2 is 1.5, the reflectance of light that is incident perpendicularly on the first principal surface 2 p is about 4%, but the reflectance can be suppressed to 1 to 2% or less, taking into account the wavelength dependency and the angle dependency, if an AR coat or anti-reflective nanostructures are formed on the surface thereof. Light other than this enters to be confined within the light-trapping sheet 51, and is used in photoelectric conversion.

FIGS. 20B and 20C are a cross-sectional view and a plan view, respectively, each showing the position at which the photoelectric conversion section 7 is attached. FIG. 20B is an example where it is arranged at the center of the light-transmitting sheet 2, and FIG. 20C is an example where it is arranged along the outer edge of the light-transmitting sheet 2. In the example of FIG. 20B, light 12 a which has been taken into the light-transmitting sheet 2 and is propagating through the inside thereof is reflected by the reflective film 11 at the end face 2 s (or 2 r) to become light 12 b propagating through the inside. Depending on the position at which it propagates, a significant distance is needed to reach the position of the photoelectric conversion section 7, and there is light that cannot reach the position of the photoelectric conversion section 7 even after repeatedly reciprocating between the end faces 2 s and 2 r a plurality of times. In contrast, in the example of FIG. 20C, since the entire outer edge portion is covered by the photoelectric conversion section 7, all the light 12 propagating through the inside can reliably reach the position of the photoelectric conversion section 7 after one reciprocation. Therefore, for the absorption loss at the light-transmitting sheet 2 and the reflection loss at the reflective film 11, FIG. 20C allows the confined light to be more effectively used in photoelectric conversion.

With the light-receiving device of the present embodiment, most of the incident light can be confined within the sheet, most of which can be used in photoelectric conversion. Therefore, it is possible to significantly improve the energy conversion efficiency of the photoelectric conversion section. The light-receiving area is determined by the area of a first principal surface p, and all of the light received by this surface enters the photoelectric conversion section 7. Therefore, it is possible to reduce the area of the photoelectric conversion section 7 or reduce the number of photoelectric conversion sections 7, thereby realizing a significant cost reduction of the light-receiving device.

FIGS. 25A to C each show a cross section and a plan view of the configuration according to a variation of the present embodiment.

The example of FIG. 25A has a configuration where a plurality of light-transmitting sheets 2 are arranged next to one another in the horizontal direction. In this example, the photoelectric conversion section 7 is arranged at the edge portion of each light-transmitting sheet. While four light-transmitting sheets 2 are arranged in two rows and two columns in the example of FIG. 25A, more light-transmitting sheets 2 may be arranged.

In the example of FIG. 25B, a plurality of photoelectric conversion sections 7 are provided on one light-transmitting sheet 2. While two photoelectric conversion sections 7 are shown in this figure, the number of photoelectric conversion sections 7 assigned to one light-transmitting sheet 2 may be three or more. As shown in FIG. 25B, a portion of the photoelectric conversion section 7 does not need to be located at the outer edge of the principal surface of the light-transmitting sheet 2.

The example of FIG. 25C uses a light-transmitting sheet 2 whose outline shape is other than a rectangular shape (e.g., a trapezoidal shape). The planar shape of the light-transmitting sheet 2 is not limited to a rectangular or square shape, but may be any of a trapezoidal shape, a parallelogram shape and other polygonal shapes. When the light-transmitting sheet 2 is used by itself, the shape of the light-transmitting sheet 2 may be any shape. Note however that if a plurality of light-transmitting sheets 2 are arranged two-dimensionally as shown in FIG. 25A, it will enhance the light efficiency to ensure that no substantial gap is formed between adjacent light-transmitting sheets 2. Thus, a plurality of light-transmitting sheets 2 may have a shape or shapes such that the light-transmitting sheets 2 can be arranged with no gap therebetween. Note however that it is not necessary that the plurality of light-transmitting sheets 2 to be arranged all have the same shape. Note that the example of FIG. 25C includes a photoelectric conversion section 7 located at the outer edge of the principal surface of the light-transmitting sheet 2, and another photoelectric conversion section 7 located in an area other than at the outer edge of the principal surface of the light-transmitting sheet 2.

Fifth Embodiment

Another embodiment of a light-receiving device of the present disclosure will be described. FIG. 21 schematically shows a cross-sectional structure of a light-receiving device 55 of the present embodiment. The light-receiving device 55 includes the light-trapping sheet 51 of the first embodiment and the photoelectric conversion section 7. The light-trapping sheet 52 of the second embodiment or the light-trapping sheet 53 of the third embodiment may be used instead of the light-trapping sheet 51. Note that the arrangement of the photoelectric conversion section 7 is the same as that of FIG. 20C.

The light-receiving device 55 is different from the light-receiving device 54 of the fourth embodiment in that a protrusion/depression structure 8 is provided on the second principal surface 2 q, with a gap between the protrusion/depression structure 8 and the photoelectric conversion section 7. The protrusion/depression structure 8 provided on the second principal surface 2 q includes depressed portions and protruding portions whose width is 0.1 μm or more and which may be in a periodic pattern or a random pattern. With the protrusion/depression structure 8, the out-of-critical-angle light 5 b′ and 6 b′ incident on the second principal surface 2 q are not totally reflected, and portions thereof travel toward the photoelectric conversion section 7 as output light 5 d′ and 6 d′ to undergo photoelectric conversion. Light that are reflected by the surface of the photoelectric conversion section 7′ are taken inside through the second principal surface 2 q of the light-trapping sheet 51 and propagates inside the light-trapping sheet 51, after which the light again travel toward the photoelectric conversion section 7 as the output light 5 d′ and 6 d′. Therefore, also with the light-receiving device of the present embodiment, most of the incident light can be confined within the light-trapping sheet, most of which can be used in photoelectric conversion. As in the fourth embodiment, it is possible to reduce the area of the photoelectric conversion section 7 or reduce the number of photoelectric conversion sections 7. Therefore, it is possible to realize a light-receiving device having a significantly improved energy conversion efficiency and being capable of cost reduction.

Sixth Embodiment

Another embodiment of a light-receiving device of the present disclosure will be described. FIG. 22 schematically shows a cross-sectional structure of a light-receiving device 58 of the present embodiment. The light-receiving device 58 includes light-trapping sheets 51 and 51′, and the photoelectric conversion section 7. The first light-trapping sheet 51, the light-trapping sheet 52 of the second embodiment or the light-trapping sheet 53 of the third embodiment may be used instead of the light-trapping sheets 51 and 51′. In the present embodiment, the fourth area 2 h may be absent in the light-trapping sheet 51′. The arrangement of the photoelectric conversion section 7 is the same as the arrangement shown in FIG. 20C, for example.

The light-receiving device 58 is different from the fourth embodiment in that the attachment is such that the end face 2 s of the light-trapping sheet 51 is in contact with the first principal surface 2 p of the light-receiving device 54 of the fourth embodiment. The light-trapping sheet 51′ may be attached orthogonal to the light-trapping sheet 51. In the light-trapping sheet 51′, the reflective film 11 may be provided on the end face 2 r, and a reflective film 11′ may be provided on a first principal surface 2 p′ and a second principal surface 2 q′ in the vicinity of the end face 2 s which is attached to the light-trapping sheet 51. The reflective film 11′ serves to reflect the light 6 b so as to prevent the out-of-critical-angle light 6 b from the light-trapping sheet 51 from leaking out of the light-trapping sheet 51′.

The light 4 incident on the first principal surface 2 p of the light-trapping sheet 51 is taken into the light-trapping sheet 51. On the other hand, light 4′ incident on the first principal surface 2 p′ and the second principal surface 2 q′ of the light-trapping sheet 51′ is taken into the light-trapping sheet 51′. Light taken into the light-trapping sheet 51′ becomes guided light 12 propagating toward the end face 2 s, since the end face 2 r is covered by the reflective film 11, and merges with the light inside the light-trapping sheet 51. Since a portion of the second principal surface 2 q in the light-trapping sheet 51 is in contact with the surface of the photoelectric conversion section 7, and the refractive index of silicon is larger than the refractive index of the light-transmitting sheet 2, the out-of-critical-angle light 5 b′ and 6 b′ incident on the second principal surface 2 q are not totally reflected but portions thereof are incident on the photoelectric conversion section 7 as the refracted light 5 d′ and 6 d′ and are converted to electric current in the photoelectric conversion section 7. The reflected out-of-critical-angle light 5 c′ and 6 c′ propagate inside the light-trapping sheet 51, are incident again on the light-receiving surface of the photoelectric conversion section 7, and are used in photoelectric conversion until the enclosed light is mostly gone.

Since the light-receiving device of the present embodiment includes the light-trapping sheet 51′ perpendicular to the light-receiving surface of the photoelectric conversion section 7, even light that is incident diagonally on the first principal surface 2 p of the light-trapping sheet 51 is incident, at an angle close to perpendicular, on the first principal surface 2 p′ and the second principal surface 2 q′ of the light-trapping sheet 51′. This makes it easier to take in light of every azimuth.

Therefore, also with the light-receiving device of the present embodiment, most of the incident light can be confined within the light-trapping sheet, most of which can be used in photoelectric conversion. As in the fourth embodiment, it is possible to reduce the area of the photoelectric conversion section 7 or reduce the number of photoelectric conversion sections 7. Therefore, it is possible to realize a light-receiving device having significantly improved energy conversion efficiency and being capable of cost reduction.

Sheets of the present disclosure are capable of taking in light over a wide area, and over a wide wavelength range (e.g., the entire visible light range) for every angle of incidence; therefore, light-receiving devices using the same are useful as high-conversion-efficiency solar cells, or the like.

While the present invention has been described with respect to preferred embodiments thereof, it will be apparent to those skilled in the art that the disclosed invention may be modified in numerous ways and may assume many embodiments other than those specifically described above. Accordingly, it is intended by the appended claims to cover all modifications of the invention that fall within the true spirit and scope of the invention. 

What is claimed is:
 1. A light-receiving device comprising a light-trapping sheet, and a photoelectric conversion section optically coupled to the light-trapping sheet, the light-trapping sheet comprising: a light-transmitting sheet having first and second principal surfaces; and a plurality of light-coupling structures arranged in an inner portion of the light-transmitting sheet at a first and second distance from the first and second principal surfaces, respectively, wherein: each of the plurality of light-coupling structures includes a first light-transmitting layer, a second light-transmitting layer, and a third light-transmitting layer sandwiched therebetween; a refractive index of the first and second light-transmitting layers is smaller than a refractive index of the light-transmitting sheet; a refractive index of the third light-transmitting layer is larger than the refractive index of the first and second light-transmitting layers; and the third light-transmitting layer has a diffraction grating parallel to the first and second principal surfaces of the light-transmitting sheet; and at least a part of the photoelectric conversion section is located along an outer edge of at least one of the first and second principal surfaces of the light-transmitting sheet.
 2. The light-receiving device of claim 1, wherein surfaces of the first and second light-transmitting layers that are located opposite to the third light-transmitting layer are parallel to the first and second principal surfaces of the light-transmitting sheets.
 3. The light-receiving device of claim 1, wherein the plurality of light-coupling structures include a first light-coupling structure and a second light-coupling structure two-dimensionally arranged next to each other on a surface parallel to the first and second principal surfaces, and the first and/or second light-transmitting layer of the first light-coupling structure and the first and/or second light-transmitting layer of the second light-coupling structure are spaced apart from each other.
 4. The light-receiving device of claim 3, wherein the third light-transmitting layer of the first light-coupling structure and the third light-transmitting layer of the second light-coupling structure are continuous with each other.
 5. The light-receiving device of claim 1, wherein the plurality of light-coupling structures are arranged three-dimensionally in an inner portion of the light-transmitting sheet at a first distance or more and a second distance or more from the first and second principal surfaces, respectively.
 6. The light-receiving device of claim 1, further comprising a transparent cover sheet opposing at least one of the first and second principal surfaces of the light-transmitting sheet with a gap interposed therebetween.
 7. The light-receiving device of claim 1, wherein a pitch of the diffraction grating is 0.1 μm or more and 3 μm or less.
 8. The light-receiving device of claim 7, wherein: surfaces of the first and second light-transmitting layers are each sized so as to circumscribe a circle having a diameter of 100 μm or less; and the plurality of light-coupling structures each have a thickness of 3 μm or less.
 9. The light-receiving device of claim 8, wherein at least two of the plurality of light-coupling structures are different from each other in terms of a direction in which the diffraction grating extends.
 10. The light-receiving device of claim 8, wherein at least two of the plurality of light-coupling structures are different from each other in terms of a pitch of the diffraction grating.
 11. The light-receiving device of claim 1, wherein: the light-transmitting sheet includes: a first area being in contact with the first principal surface and having a thickness equal to the first distance; a second area being in contact with the second principal surface and having a thickness equal to the second distance; a third area sandwiched between the first and second areas; and at least one fourth area provided in the third area and connecting the first area and the second area to each other; and the plurality of light-coupling structures are arranged only in the third area excluding the at least one fourth area; and an arbitrary straight line passing through the fourth area is extending along an angle greater than a critical angle, which is defined by the refractive index of the light-transmitting sheet and a refractive index of an environmental medium surrounding the light-transmitting sheet, with respect to a thickness direction of the light-transmitting sheet.
 12. The light-receiving device of claim 1, wherein in at least one of the plurality of light-coupling structures, thicknesses of the first and second light-transmitting layers are decreased toward an outer edge side away from a center of the light-coupling structure.
 13. The light-receiving device of claim 1, wherein in at least one of the plurality of light-coupling structures, a protrusion/depression structure whose pitch and height are ⅓ or less of a design wavelength is formed on one of surfaces of the first and second light-transmitting layers that are in contact with the light-transmitting sheet, the first principal surface, and the second principal surface.
 14. The light-receiving device of claim 1, wherein the refractive index of the first and second light-transmitting layers is equal to a refractive index of the environmental medium.
 15. The light-receiving device of claim 1, further comprising another light-trapping sheet, wherein the photoelectric conversion section is provided on the first principal surface of the light-trapping sheet; and an end face of the other light-trapping sheet is connected to the second principal surface of the light-trapping sheet. 